1. Field of the Invention
In general, the present invention relates to a flat field optical scanning system that can produce scan lines at high speeds, high resolution, and large formats. More particularly, the present invention relates to a flat bed optical scanning system with a polygon scanner having a fluid film bearing that accomplishes correction of repeatable and non-repeatable cross scan and in-scan errors such that the associated errors are reduced to negligible levels as the scanned energy of the laser beam is directed to its intended surface.
2. Description of the Prior Art
In a scanning system, the light source is typically a continuous wave gas laser or a laser diode. The laser beam produced by one of these devices is typically first collected by lenses that condition the beam to be either collimated or focused, is then deflected by the scan optic and then focused onto an imaging surface. Conditioning optics may also be part of the scan optic, or placed between the scan optic to influence focusing of the beam, or passive correction of scan curvature, or bearing cross-scan wobble correction using anamorphic optical elements.
Notwithstanding all of the above alternatives, the deflected light beams are scanned into a line that, when combined with a separate linear transport mechanism operating in a direction orthogonal to the scan line, produces a two-dimensional image made up of a series of small dots or pixels. Discrete picture elements (pixels or dots) are created by modulating the laser light source. A laser diode may be modulated directly by varying the applied power. A continuous wave gas laser may be modulated by use of outside means such as an acousto-optic modulator. The imaging surface itself may be either flat or curved, depending upon the optical design configuration and application of the particular scanning device. In a typical cylindrical imaging application, for example, a flexible photo-sensitive material is first loaded onto the inside or outside of the cylindrical surface matched in radius to the curvature of the scanned energy. The optical scanning device is then precisely moved along precision made rails at a constant speed along the center axis of the drum. The photo-sensitive material that is guided or attached to the cylindrical surface is scanned (or exposed) by the light beams reflected by the rotating scan optic of the optical scanning device.
Cylindrical imaging systems are inherently simple since the curvature produced by the operation of the scanner is corrected by the curvature of the cylindrical locating surface. In addition, the duty cycle or scan efficiency can approach 100% if the fall 360 degrees of the cylindrical surface is utilized. Cylindrical imaging systems do, however, have very significant limitations. The surface to be scanned (or exposed to the light) must be flexible and the length of the surface is limited by the length of the drum. While it may be possible to make the cylindrical surface very large in radius and very long, as a practical matter, cost and accuracy factors become increasingly prohibitive if the radius and length of the cylindrical surface exceed 18" and 48", respectively. Generally, in order to take advantage of the inherent simplicity of the cylindrical imaging configuration, only one scan line can be produced per revolution of the scanner. Thus the scanner must rotate at a high rate of speed to achieve a high scan rate. At high rates of speed, problems with noise, windage and deflection of the optical surface are increasingly problematic.
The rotatable scan optic in the cylindrical imaging context typically consists of a single mirror, an assembly of more than one mirror, or a glass prism with one or two reflective surfaces. Generally one scan line per revolution of the scanner is produced, although Kramer, U.S. Pat. No. 4,786,126, teaches a design whereby two scan lines per revolution may be produced. The rotatable scan optic may be mounted on a spindle supported by ball bearings or by a fluid film bearing. A fluid film bearing utilizes a gas or oil to separate and lubricate sliding surfaces and may be externally pressurized or generate its own internal pressure, referred to as self-acting. Fluid film bearings are superior to ball bearings in terms of rotational accuracy, repeatability, and high rotational speeds.
The ideal cylindrical scanning system will be capable of a very high rate of scan, high resolution, and high scan efficiency while maintaining these qualities for large sizes of reproductive media. For example, see U.S. Pat. No. 5,610,751, to Sweeney et al., that teaches a self-acting gas bearing enclosed in a spherical windowed housing used to establish an accurate, high speed, low noise, lubrication-free, contamination protected, long living, scan motor and optical assembly. Self-acting gas bearings are preferred for high performance cylindrical imaging applications where exceptional accuracy, high rotational speeds, and low velocity jitter are required. Since cylindrical imaging systems utilize, as a general rule, one scan line per revolution, the accuracy and repeatability in the registration of adjacent scan lines is solely dependent on the accuracy and repeatability of the bearing and the motor velocity control system. The use of a gas film bearing provides an accurate platform that has become the predominant technology for cylindrical imaging systems.
To summarize, cylindrical imaging systems are inherently simple, but lack the general utility of flat field systems, since they must reconcile large cylindrical surfaces and flexible reproductive materials. As demonstrated above, it is also increasingly problematic to design and operate a single scan line per revolution scanner as the scan rate is increased, aperture size and resolution is increased, and imaging format size is increased.
In a typical flat field imaging application, a photo-sensitive material, for example, is moved at constant speed by a capstan roller or other linear conveyor means, fed from continuous rolls or precut cassettes of stacked material to present the material to a stationary optical system for scanning. Alternatively, stationary photo-sensitive material may be imaged by translating the optical system. Flat field systems have superior general utility over cylindrical imaging systems because the surface that is to be imaged onto or inspected is not required to be finite in length, need not be flexible and may thus be fed continuously at great speed. Imaging of stiff metal plate materials and inspection of electronic components require the use of flat field scanning systems.
The scan optic in the flat field imaging context typically consists of a resonating or rotating single facet mirror, or an assembly of two or more mirrors, or a glass prism consisting of one or two reflecting surfaces. It is also common to have a rotating polygon or hologon having multiple reflective or refractive facets symmetric to a central rotating axis.
Flat field systems require the use of additional conditioning optics to flatten the curvature produced by the rotation of the scanner as the beams are swept into an arc. This correction is commonly known as f-theta correction. F-theta curvature error can otherwise be corrected by imaging onto a cylindrical surface having a matched radius of curvature as discussed previously. F-theta curvature may also be corrected by refractive (lenses) or reflective (mirrors) means. F-theta conditioning optics for large format, high resolution applications, typically cannot perform adequately for angles of scan much greater than 22.5 degrees of scanner rotation (45 degrees of optical scan angle) out of a total of 360 degrees. This results in an effective scan efficiency of approximately 12% for a flat field, rotating, single facet scan optic that can produce only one scan line per revolution compared to up to 100% scan efficiency that can be achieved with the cylindrical approach. Alternatively, the duty cycle of a single facet resonant scanner is improved to 30-35% at the expense of large variations in scan velocity which must be corrected for in modulation of the laser beam. This loss of scan efficiency is a significant disadvantage since the rate at which the laser must be modulated and the instantaneous power of the laser must both be increased significantly compared to the cylindrical imaging case for a given rate of scan due to the dramatic reduction of scan efficiency. If a multi-facet device is used, such as a polygon or hologon, each facet can service the 22.5 degree acceptance angle of the f-theta optics thus the scan efficiency for a polygon or hologon scanner for flat field systems can be as high as 50-80% if 8-12 facets are used. However, the use of multiple scan surfaces on the scan optic to multiply the effective scan rate, as is the case for both the polygon or hologon scanners, produces undesirable side effects unique to these types of scanners. The use of a multi-faceted holographic rotating scanner has unique advantages with respect to cross-scan error and scan efficiency compared to a rotating polygon scanner. Its principal disadvantage is the requirement for a laser having refined wavelength stability. Such a device also cannot be used for alternative wavelengths without change of design and is completely unsuitable for simultaneous multiple color scanning.
Multi-facet scanners in general, and polygon scanning systems in particular, have inherent limitations in that corrections must be made for relative errors in the facets of the polygon that deflect laser energy to create a scanned line. As the object of scan is moved away from the scanner, as is the case for large imaging formats, and as the resolution is increased, very small errors in the facets of the polygon in relation to its rotating axis and variation in the rotating velocity can produce noticeable errors. These errors are commonly known as cross-scan and in-scan related errors.
It is known to use anamorphic correcting optics that can correct for cross-scan errors. These optics become increasingly difficult and expensive to design and manufacture as the size of the imaging format increases. Typically, the anamorphic cross-scan correction approach is cost and performance limited for scan widths greater than 14" and resolution greater than 2,000 dots per inch. Though it is known to use anamorphic correction with flat field system widths of as large as 26", the cost for such a system is prohibitive. An additional disadvantage of the anamorphic approach is that the use of multiple wavelengths of light, especially at the same time, is restricted.
A well known alternative to the anamorphic approach is the use of an active cross-scan correction approach that simplifies the optical design significantly by eliminating the need for anamorphic correction elements that would be very large and very expensive for large format operations and that restricts the goal for having a system that is polychromatic. Besides the cross-scan error, the in-scan error component is also of equal importance for both cylindrical and flat field imaging. For a rotating optic producing a single scan line per revolution, in-scan errors are solely attributable to the accuracy and repeatability of the rotating velocity. To a significant extent, the accuracy need not be as good as the repeatability. For imaging systems, small variations in velocity that repeat at the same place from one scan line to the next do not produce an error that is noticeable to the human eye. Multiple facet scanners such as the rotating polygon require more refined accuracy of velocity control as well as repeatability since each facet represents a fraction of the total rotation and the period of each fraction must be nearly identical. Otherwise in-scan related errors become readily visible.
In the use of a polygon scanner, whether passive or active means are used to correct for cross-scan error, the rotatable scan optic is commonly mounted on a shaft supported by a bearing assembly normally including radial and thrust support components. Fluid film bearings are used in scanners of all types where exceptional accuracy, high rotational speeds, and low velocity jitter are required.
It is essential to the present discussion, in view of the lack of common terminology within the optical scanning and precision instrument industry in general, to distinguish between non-repeating and repeating sources of error. Non-repeating or random errors are difficult to isolate and correct. Sources of non-repeating errors in the context of optical scanning devices are often caused by aerodynamic or windage effects on the rotating optic or errors in the bearing supporting the scan optic that defines the axis of rotation of the scanner.
Repeating errors recur in the form of periodic and predictable patterns and can generally be measured, and in many cases compensated for, using feedback error correction.
Within the flat field applications, it is well known that the rotating polygon scan optic has greater potential for speed and efficiency compared to the use of the resonating scanner or single facet rotary scanner. The polygon scan optic is superior because for each rotation thereof, a polygon scan optic having "n" number of facets produces "n" number of scan lines, whereas for the single facet rotary scan optic or resonating scanner, for each revolution of the scan optic, one scan line is produced. Thus, to obtain a high resolution image on a given imaging surface in as short amount of time as possible, it is desirable to maximize the scan rate of the scan optic. In addition to the rotating polygon scanner operating at a greater scan rate, it can use a larger aperture, and the reflecting surfaces of the polygon scan optic are less susceptible to distortion from centrifugal body forces by comparison to single facet, rotating or resonant scanners.
However, this potential of the polygon scan optic cannot be realized unless scanning errors unique to the polygon scanning device can be reconciled in the design of the overall scanning system. In this regard, it is important to note the image producing quality and productivity of a polygon optical scanning device depends largely on its precision and on the scan rate and scan efficiency of the device's polygon scan optic, respectively. Precision is necessary to achieve higher resolutions, and a higher scan rate and efficiency are necessary to generate the high resolution images faster.
One significant scanning error common to polygon scanning devices is "cross-scan" error. Cross-scan error specifically refers to errors in the placement of scan lines in a direction perpendicular to the lines being scanned. The polygon cross-scan error phenomena is the result of one or more of three separate sources of error. The first source of error is the accuracy of the rotating axis of the motor-driven bearing assembly on which the polygon is mounted or integral to. This type of error is generally non-repeating in nature. The second source of error is the parallelism of the true rotating axis of the spindle and the virtual axis of the polygon. This type of error is generally repeating in nature. The virtual axis is defined as that axis best fitted to the relative angles of each of the polygon facets. The third source of error is the relative angle errors of each facet to the defined best-fit virtual axis. This error is also repeating in nature. All of these errors sum together resulting in composite cross-scan error.
Because of the above noted superior advantages of flat field scanning compared to cylindrical scanning, it has been the subject of many efforts in the prior art to develop polygon scan optic systems with various error correction schemes. For example, U.S. Pat. No. 5,365,364, to Taylor, discloses and teaches the design of an all reflective flat field imaging system with multiple facets, having high scan efficiency, suited for use at numerous operating wavelengths of light enabled by the all reflective design. The Taylor device is taught as being aerodynamically smooth to reduce bearing perturbation due to windage and, accordingly, it has a potential for improved scan rate enabled by improved scan efficiency.
U.S. Pat. No. 5,281,812, to Lee et al., discloses and teaches a flat field imaging system with an f-theta lens that utilizes a closed loop control system to correct for the repeating and non-repeating cross-scan errors of a polygon scanner in real time by implementation of a novel piezoelectric driven mirror. Lee et al. teach the limitation of single faceted scanners in the flat field context, the problems with acousto-optic modulators to influence cross-scan correction, and the cost and implementation limitations of the use of anamorphic cross-scan correction optics. Lee et al. also teach the fundamental limitations of the natural frequency of the spring-mass system embodied in the implementation of a mirror driven at high frequency by piezoelectric actuators.
U.S. Pat. No. 5,247,174, to Berman, discloses and teaches a flat field imaging system having an f-theta lens that utilizes a closed loop control system to correct for the repeating and non-repeating cross-scan errors of the polygon scanner in real time by attaching the end of a fiber optic coupled to a gas laser onto a piezoelectric actuator and correcting the errors of the polygon scanner by moving the laser beam source.
U.S. Pat. No. 4,441,126, to Greenig et al., discloses and teaches a beam deflection system having an acousto-optic modulator connected to a lens located between the laser and the polygon scan optic. The Greenig et al. reference discloses a sensor and bridge circuit to sense the position of the beam scan and error value for each facet of the scan optic. The values are averaged and a correcting signal and voltage adjustment are supplied to adjust the balance of the bridge to drive the average value toward a reference value.
U.S. Pat. No. 4,054,360, to Oosaka et al., discloses and teaches an improved method and apparatus for removing scanning error associated with the lack of perfect parallelism in a rotating polyhedral mirror. Oosaka et al. teach the use of an incident beam directed along a vertically independent optical patch and brought back into incidence on the same mirror deflection point to eliminate the error in parallelism without interfering with the horizontal scanning of the reflected beam.
The examples of prior art mentioned above focus on reduction of scanning errors in polygon scanning systems by utilizing active and passive means to reduce the repeating errors, but fail to disclose, teach or suggest the use of a fluid film bearing for rotating the polygon scan optic to reduce the non-repeating errors. The accuracy of these systems are limited by the accuracy of the bearing system which typically induces significant non-repeating errors. Alternatively, if a fluid film bearing, or more specifically, a self-acting gas bearing is utilized to reduce non-repeating errors in conjunction with active or passive means to correct for repeating errors, then a significant reduction of composite errors is realized and system accuracy improved.
Moreover, the inherent accuracy and repeatability afforded by a fluid film bearing also enables the use of an open loop control system for active correction of repeatable scanning errors. Open loop correction techniques are not practical for polygon scanning applications unless the non-repeating component of the system errors is reduced to negligible levels.
Active and passive error correction techniques are well known in the art. Active cross-scan correction implies that the errors are continuously tracked or mapped in the operation of the system and some sort of mechanism within the design continuously implements an equal and opposite error to that of the composite error of the rotating polygon sub-system. Prior art techniques include the use of an acousto-optic modulator in the path between the laser source and the rotating polygon. The acousto-optic modulator can be used to re-direct the beam at precisely the opposite angle that each polygon facet requires to achieve negligible cross-scan error. An alternative approach that has been used is to tilt a mirror in the system by use of a device such as a piezoelectric actuator or voice coil type of actuator. Piezoelectric actuators, voice coil actuators, and many other types of actuators of similar vein are broadly classified as electro-mechanical actuators.
A second undesirable source of error in optical scanning systems results from "in-scan" error. In-scan error specifically refers to errors in the placement of scan lines in a direction parallel to the lines themselves. In-scan error can also cause cross-scan errors since velocity variations of the polygon scan optic result in placement errors on the scanned media. Like cross-scan error, in-scan error is also the result of several components both repeatable and non-repeatable. The first source of in-scan error is related to the accuracy of the rotating axis of the bearing. If the rotating axis of the bearing is not perfectly aligned with the rotating axis of the polygon scan optic, there will be some repeatable error. The second error is the relative height of each of the polygon facets to the axis of rotation of the bearing. If the bearing has a nearly perfect axis of rotation, as is the case for a fluid film bearing, facet height errors are isolated to the manufacture of the polygon and its registration to the bearing axis. Facet height errors result in length variations from one scanned line to the next. The error pattern is repeatable with each fall rotation of the polygon. Facet height errors can be reduced to negligible levels by controlled manufacturing processes applied to the manufacture of the polygon. This error may also be corrected by inducing small variations in the rotating velocity of the scanner.
The third error affecting in-scan error relates to the accuracy of the speed control feedback control system that governs the rotational velocity of the polygon. The in-scan error problem for the polygon scanning system is most significant for high resolution, flat field imaging systems. This is because the distance from the polygon to the reproductive media tends to be large and incremental velocity errors less than 1/1,000,000 of a revolution of the scanner can produce noticeable in-scan related artifacts. Single facet scanning systems can tolerate much greater velocity variation so long as the velocity profile repeats from one revolution of the scanner to the next. As an example, typical single facet, air bearing, rotary scanners have incremental velocity errors on the order of 5-10 parts per million/revolution (PPM/rev) but repeat at any point of interest in the scan to a precision of less than 1.0 PPM/rev. Since the polygon scanner generates a multiplicity of scan lines per revolution, incremental velocity variation cannot be tolerated.
It is important to distinguish between "short-term" and "long-term" errors. Short-term errors are defined herein as errors that appear within fractions of a revolution up to several thousand revolutions of the rotating scanner. Long-term errors are defined as errors that occur over more than several thousand revolutions of the rotating scanner, or otherwise defined over a significant period of time or number of scan lines. Short-term and long-term errors can both be repeating and non-repeating in form. Very small short-term errors are known to produce imaging anomalies such as "banding". In general, larger errors spread over longer periods of time can be tolerated. Actual tolerances depend on many factors unique to a particular imaging application including error spacial frequency, image contrast, and overall image distortion.
In summation, the prior art has yet to disclose or teach singularly, or in any combination, and there continues to be a significant need for, a large format, flat field, high resolution, high speed optical scanning system, at a cost that makes such a device practical and usable in a significant number of applications. More particularly, the prior art is lacking and there remains a need for a flat field scanning system that utilizes a polygon scanner with a fluid film bearing to reduce non-repeating scanning errors, and has an open loop control system for active correction of repeatable scanning errors, and is capable of producing large formats at high resolution and high scan rates.